Sight glasses can be fitted to vessels, chambers and pipes to enable an operator or an optical sensor to observe the interior of the vessel, chamber or pipe, for example to view a liquid level or behaviour. A sight glass generally comprises a (usually circular) pane of toughened glass within a metal surround. The sight glass can be installed to a vessel by welding, bolting or clamping the metal surround to an opening in the vessel, chamber or pipe.
While sight glasses are commonly used in many industries, sight glasses are not used in natural gas process and distribution systems. The reason for this is that a sight glass failure within the high pressure environments of these systems would be catastrophic and pose a large risk for oil and gas industries.
Some attempts have been made to minimise the impact of a sight glass failure. For example, it has been proposed that a secondary sight glass of the same diameter as the primary sight glass could be mounted directly behind the primary sight glass. However, there is a problem with this—if the primary sight glass fails under pressure, the debris from the failed primary sight glass would impact the secondary sight glass and cause it to fail as well. Some previous attempts at improving the safety of sight glasses are described in CN202778414, CN202580005, U.S. Pat. No. 4,415,235, GB1487056, GB909527 and EP2159619.
Embodiments of the present invention seek to address these problems, and to improve safety when using sight glasses in high pressure systems.